This invention relates to a controllably bendable tube assembly, especially a borescope or endoscope of the type having a cable-actuated hollow steering section.
An endoscope is generally characterized as an elongated flexible tube, i.e. an insertion tube, with a viewing head at its distal or forward end, and a control section at its proximal end for controlling or steering the distal end. In such an endoscope, a bendable tube steering section is situated at the distal end adjacent to the viewing head. One or two pairs of control cables extend through the bendable tube section and the remainder of the insertion tube and connect with a steering control mechanism in the control section. One or both pairs of these cables are differentially displaced for bending the steering section to facilitate the inspection of an object.
An endoscope is typically inserted into a body cavity of a patient for visual investigation of tissues within the cavity. For example, an endoscope can be inserted into the colon, stomach, or lung of a patient. Because the esophagus, bronchii, and colon are narrow, tortuous passageways, the steering section must be bent rather precisely, and as close to the viewing head as possible, in order to obtain the necessary penetration without damaging the patient's tissues. It is most desirable that both the cable tension be limited and that cable slack be minimized.
A borescope is a similar device, but intended for visual inspection of a mechanical assembly, such as a jet engine or turbine, where it would be difficult or impossible otherwise to the assembly's internal elements. The borescope needs to be insertable into narrow tortuous passageways, and must observe similar steering and bending considerations.
A number of types of steering mechanisms are known. For example, helically coiled strips are employed in endoscopes or borescopes as described in U.S. Pat. Nos. 3,610,231 and 3,739,770. Steering sections formed of thin-walled cylindrical segments or bands that are joined by means of pins or bifurcations, or other similar articulations such that the segments are rockable on one another, are described in U.S. Pat. Nos. 3,583,393; 3,669,098; 3,799,151; and 4,347,837. U.S. Pat. No. 3,557,780 describes an endoscope steering section formed of two flexure portions, with two sets of control wires. Stays or flexible backbone members of various lengths control the degree of curving and the location of the curvature on the steering section.
An endoscope described in the U.S. Pat. No. 3,799,151, has cylindrical segments articulated in one plane or another plane as required to select the amount and direction of bending of the endoscope steering section.
For those steering sections that are bendable in two planes, a significant amount of cable slack is typically included so that the steering cables for one plane do not bind when the steering section is bent in the other plane. Some cable slack is also included to accommodate cable tightening due to coiling and bending of the insertion tube through which the steering cables pass. The endoscopes and borescopes are usually coiled for shipping and storage, but are generally straight when in use. This coiling can place large stresses on the cables, leading to cable failure.
In a steerable endoscope or borescope, opposing steering cables are displaced to deflect its distal tip. These cables are differentially displaced. That is, as one cable is pulled away from the bending section, the other moves towards the bending section. However, the motion of the one cable is not normally the exact opposite of the motion of the other. Coiling of the insertion tube can result in the tensioning, at the same time, of both cables of an opposed pair. This tensioning produces friction and high forces that can damage the steering section or cables, leading to early failure. Adding cable play or slack can alleviate this problem, but can create other problems, such as imprecise steering. Moreover, large steering knob movements are then required for deflection of the endoscope or borescope tip.